database_exists throws error if username and db name are different for postgresql

[AramayisO]

In my test environment, I have a user named "user" that only has access to a database named "test". If I run
database_exists("postgresql://user:password@localhost/test"), I get the following error: sqlalchemy.exc.OperationalError: (psycopg2.OperationalError) FATAL: database "user" does not exist.

It looks like this behavior results from this line of code introduced in 5d99747

sqlalchemy-utils/sqlalchemy_utils/functions/database.py

Line 461 in 69f612a

database, url.database = url.database, None

With url.database set to None, the code assumes we are connecting to a database with the same name as the username.

[fmigneault]

I'm also seeing this problem.
I can confirm that reapplying the url.database name to the original value (ie: test in provided example) correctly finds the expected database, and with None enforced here, it looks for user database.

[fmigneault]

duplicate of #462
fixed by #463

[dpb-bah]

Can confirm this is an issue. Had to revert to 0.36.7.

[redspart]

I see this observe this behavior:

• Python3.6.9 running version 0.36.8 gets the error above
• Python3.6.9 running version 0.36.7 does not get this error
• Python3.6.12 running version 0.36.8 does not get this error

[sfc-gh-pkommini]

I see this error on python 3.8.6 and 0.36.8 but don't see it on 0.36.7. Although, I think it has less to do with the python version and more to do with the sqlalchemy-utils library version

[tanj]

This commit 5d99747 needs to be reversed.

PostgreSQL needs to connect explicitly to the postgres database if connecting with a user other than postgres to be able to query pg_database

Current test cases only connect with postgres user, so seem to pass be allow for this failure.